Genistein and daidzein are estrogenic isoflavones found in soybeans. Although they are much less potent than estradiol, infants fed soy formula have serum concentrations that are up to 4 orders of magnitude higher than estradiol. Anatomic, sonographic, and other biochemical evidence of estrogen exposure has not been looked for in infants fed soy. Whether such widely used products do or do not have clinically detectable estrogenic activity in infants, the group with the highest exposure per kilo, is important clinically;in addition, it may be a crucial test of the ability of laboratory measures of estrogenicity to predict actual endocrine response in humans. We have done the field work for pilot studies to examine the natural history of estrogen responsive physiology in the newborn, and have used the results to develop a longitudinal study of the possible estrogenic effects of soy formula on infants. Using samples from this project, we are collaborated with the Centers for Disease Control to see if perchlorate, a contaminant of the US water supply, lowers thyroid hormone levels in infants as it does in adult females. We have reported in abstract form that hogher perchlorate exposure is associated with higher thyroid stimulating hormone level but not lower T4. This finding needs further research.